Cable television (TV) systems utilize an architecture called Hybrid Fiber Coax (HFC), as illustrated in FIG. 1. The HFC architecture relies on a mixture of fiber optic technology and coaxial cable-based transmission technology. The cable TV system is comprised of a central facility called a head-end office 1, where central equipment controlling much of the cable system resides, optical node 6, coaxial (coax) distribution network 7, and equipment at customer premises 8. The head-end office 1 can serve a very large number of customers, often an entire city or a metro area. The head-end office 1 uses fiber optic cables to cover long distances between its location and optical node locations. Fiber optic medium is well suited for this portion of the network due to its ability to propagate optical signals across long distances with small signal power losses. The coax portion of the network generally covers short distances due to its relatively high signal power losses. A network of radio frequency (RF) power amplifiers is used to boost the RF signals power along the coax distribution network 7 to provide uniform RF power levels across the covered area. HFC coaxial distribution network is a shared medium that is used simultaneously to carry both the upstream and the downstream signals by employing Frequency Division Multiplexing (FDM).
As illustrated in FIG. 2, a small portion of the lower frequency spectrum is typically assigned to the upstream channels (F1 to F2), followed by guard band (F2 to F3) and then followed by an upper frequency spectrum assigned to the downstream channels (F3 to F4). This frequency band splitting leads to a substantial asymmetry in the upstream versus the downstream frequency spectrum allocation, where the downstream frequency spectrum is many times wider than that of the upstream spectrum. Furthermore, the lower frequency spectrum is more susceptible to external noise ingress than the upper spectrum and therefore can only supports lower orders of Quadrature Amplitude Modulation (QAM), leading to lower overall data capacity efficiencies per channel.
The following are descriptions of signal flow from the head-end office 1 toward the customer premises 8, also referred to as “Downstream” signals, in a cable TV HFC system. Video feeds from various sources 2, such as satellite receivers, fiber optic cables or microwave links, are funneled at the head-end office 1 to a network of RF channel combiners 3. Internet data communications is managed by a central equipment called Cable Modem Termination System (CMTS) 12. The CMTS 12 bridges a large number of cable modems at customer premises 8 and the Internet 13. This bridging function is achieved by broadcasting encrypted data and control packets to the connected cable modems 9 and allocating time slots for individual cable modems 9 to access the shared upstream path. The CMTS 12 features downstream RF ports, each connected to RF channel combiner 3. The combined video channels and CMTS data channels are then converted to optical signal by optical transmitters 4, and the optical signal is then launched into fiber optic cables 5 connecting the head-end office 1 with optical node 6. At the optical node 6, the optical downstream signal is converted to an electrical signal, amplified, and sent toward the customer premises 8 via the coax distribution network 7. At the customer premises 8, coaxial cables connect to a set top box 15 or directly to TV sets. The coaxial signal is also connected to the cable modem 9, where data packets destined for its specific customer are captured and sent to a computer 10 via a local computer port 14.
The following are description of the signal flow from the customer premises 8 toward the head-end office 1, also referred to as “Upstream” signals, in a cable TV HFC system. The upstream path resources are shared among a large number of customers, and therefore a critical role of the CMTS 12 is to dynamically allocate time slots and frequency channel/s for each customer premises equipment and thereby avoid collisions. Data packets generated by the customer's computer 10 are received by the cable modem 9 via its computer port 14. These RF modulated data packets, ultimately destined for the Internet 13, then are transmitted on the upstream channels of the coax distribution network 7 by the associated cable modem 9 on its allocated time slots and frequency channel/s. Upstream data signal traversing over the coax distribution network 7 are received by the optical node 6, converted to optical signal, and sent to the head-end office 1 via fiber optic link 5. At the head-end office 1, optical upstream signals containing data packets from numerous cable modems 9 are then converted to electrical signals via optical receiver 11 and delivered to the CMTS 12 upstream port. The upstream data packets are then processed by the CMTS 12, their destination address modified according to switching and routing tables, and then sent to the Internet 13.
FIG. 3 illustrates a cable TV Fiber to the Home (FTTH) system, where multiple customers receive their video signals and communicate with the Internet via cable modems. The cable TV FTTH system is comprised of the head-end office 1, fiber optic cables, small optical nodes 6 on the side of each customer building, and customer premises equipment. The head-end office 1 can serve a very large number of customers, often an entire city or a metro area. The head-end office 1 may also serve both HFC & FTTH customers. Cable TV FTTH systems rely on transporting a number of signals bidirectionally over a single fiber optic cable by employing a well-established technology called Wavelength Division Multiplexing (WDM), whereby each signal type is transmitted using a unique optical wavelength, also referred to as “color”. Cable FTTH system places the optical node 6 physically very close to, or on the side of the customer building that it serves. This arrangement brings the fiber optic cable into or near the customer premises 8, and hence, this architecture is known as a Fiber To The Home (FTTH) system.
The following are descriptions of signal flow from the head-end 1 toward the customer premises 8, also referred to as “Downstream” signals, in a cable TV FTTH system. Video feeds 16 from various sources, such as satellite receivers, fiber optic cables or microwave links, are funneled at the head-end office 1 and optically transmitted on a specific wavelength λ1. Internet data communications is managed by a CMTS 12. The CMTS 12 bridges a large number of cable modems 9 at customer premises 8 and the Internet 13. This bridging function is achieved by broadcasting encrypted data and control packets to the connected cable modems 9 and allocating time slots for individual cable modems 9 to access the shared upstream path. The CMTS 12 features downstream RF ports, each connected to an optical transmitter 4 that converts the CMTS downstream signal to an optical signal with wavelength of λ2. The video channels and CMTS downstream data channels are then optically combined by optical combiner 19. A wavelength division multiplexer/de-multiplexer (WDM Mux/D-Mux) 17 then launches the combined downstream optical signals into long distance fiber optic cables 5 connecting the head-end office 1 with optical splitter 18. Optical splitter 18 is used to split the optical signal present on optical cable 5 into N number of optical signals, each connected to a specific FTTH optical node 6. Optical splitter 18 performs the opposite in the upstream direction, combining the optical signals transmitted by the various optical nodes 6 and launches the combined upstream optical signal into optical cable 5.
The optical node 6 at each customer premise 8 then converts optical signals to electrical signals in the downstream direction and converts the electrical signals into optical signals in the upstream direction. At the optical node 6, the optical downstream signal which contain both video channels and data channels is converted to an electrical signal, amplified, and sent toward the customer premises 8 via a coax cable. At the customer premises 8, the coax cable connects to a set top box or directly to a TV set for video reception. The coax signal is also connected to cable modem 9, where downstream data packets destined for its specific customer are captured and sent to a local computer port 14.
The following are description of the signal flow from the customer premises 8 toward the head-end office 1, also referred to as “Upstream” signals, in a cable TV FTTH system. The upstream path resources are shared among a large number of customers and therefore a critical role of the CMTS 12 is to dynamically allocate time slots for each customer premises equipment and thereby avoid collisions. Data packets generated by the customer's computer 10 are received by the cable modem 9 via its computer port 14. These data packets, ultimately destined for the Internet 13, are then transmitted on the upstream channels of the coax cable by the associated cable modem 9 on its allocated time slots. Upstream data signal traversing over the coax cable are then received by the optical node 6, converted to optical signal with wavelength λ3 and sent toward optical splitter 18, where the optical upstream signal is sent toward the head-end office 1 via fiber optic link 5. At the head-end office 1, optical upstream signals containing RF modulated data packets from numerous cable modems 9 are demultiplexed by the Mux/D-Mux 17, converted to electrical signals via optical receiver 11, and delivered to the CMTS 12 upstream port. The upstream data packets are then processed by the CMTS 12, their destination address modified according to switching and routing tables and sent to the Internet 13.
Extending the optical fiber deeper into the network 20 edge reduces or eliminates altogether RF amplifiers which enhances system capacity but at the same time increases the number of optical nodes substantially. As can be observed in FIG. 4, as the number of optical nodes 6 is increased, the number of optical spans connecting the optical nodes 6 to the head-end office 1 increases proportionally. Two existing techniques attempt to address upstream aggregation: optical node daisy chaining, and Radio Frequency over Glass (RFoG).
FIG. 5 illustrates a daisy chaining technique for upstream aggregation. In this technique, aggregation of the upstream channels from multiple optical nodes 6 at the edge of a fiber deep network 20 is accomplished by daisy chaining optical nodes using fiber optic, resulting with only the last optical node in the chain sending a single optical signal to the head-end office 1. This reduces the need and cost associated with increasing the long-haul portion of the optical transport connecting optical nodes 6 to the head-end office 1. However, prior art daisy chaining techniques present several disadvantages: single point of failure; complicated management; and noise funneling.
Daisy chain systems are very susceptible to a single point of failure since RF samples from optical nodes 6 must traverse through multiple other optical nodes 6A before reaching the optical receiver 11 at the head-end office 1. A failure in any optical node 6A in the chain, or a failure of any fiber span in the chain, will result in islands of optical node segments 6 that are disconnected from the optical receiver 11. The failure rate is directly proportional to the size of the daisy chain system, where an increase in the number of optical nodes and optical spans in the chain increases the likelihood of failure. This therefore limits the practical number of optical nodes that can be in the daisy chain system.
Daisy chain systems further have the disadvantage of requiring complicated management. Managing and monitoring large numbers of optical nodes 6 involve several processes, including the capability of the system to dynamically discover network topology, assign an address to each optical node, and establish resilient communications with each optical node even in the presence of common errors in the communication links. Daisy chained architecture requires complicated and often burdensome resources to achieve these basic management and monitoring tasks. Furthermore, in an event of equipment or plant failure, some or most monitoring functions cease to function at most inopportune times.
Daisy chain systems further have the disadvantage of noise Funneling. The digital summing of RF samples is not noiseless. Every time the number of nodes in a digital daisy chained system is doubled, the noise floor on the RF output of the system will be increased by 3 dB. This limits the practical number of optical nodes that can be in a daisy chain system.
FIG. 6 illustrates prior art Radio Frequency over Glass (RFoG) technique for upstream aggregation. RFoG aggregates the upstream channels from multiple optical nodes 6 at the edge of fiber deep network 20. A 1×N passive optical splitter 18 is used to connect multiple optical nodes to the head end office 1. To reduce noise funneling of multiple connected optical nodes, the optical nodes 6 are equipped with burst type analog laser transmitters. The laser on-board the optical nodes 6 are turned-on only during the duration that a RF signal generated by Customer Premises Equipment (CPE) is detected. As illustrated in FIG. 7, these RF burst signals are composed of a preamble section 21 followed by modulated data sections 22. Furthermore, the preamble section 21 of the burst is shaped in a way that resembles function of sin x/x. This signal shaping is essential to allow rapid signal locking by the burst receiver circuit of the CMTS 12 or other burst QAM receivers. The RFoG technique, however, presents several disadvantages: loss of preamble; limited number of optical nodes; lower upstream link performance; and optical beating interference.
RF burst detection circuit in the optical node 6 is programmed to define “Start of Burst” only after the level of RF signal reaches certain predetermined amplitude (threshold) and the burst duration is longer than predetermined period. As illustrated in FIG. 8, this leads to a delay from the start of the RF burst to turning-on the laser. Once the laser is turned-on, additional delay occurs until the laser reaches its full power. Because of these delays some portion of the preamble signal is lost. The partial loss of preamble section 21 can lead to malfunction at the burst receiver at the CMTS 12, and therefore, RFoG systems must use longer preamble fields to compensate. This results in a lower throughput of the system.
The RFoG technique further has the disadvantage of limiting the practical number of optical nodes. RFoG type optical nodes include an analog laser that converts the detected upstream RF burst to optical bursts destined to reach the head-end office 1. Laser turn-on time delay is a function of multiple parameters, among them laser optical power output at “off state”. The lower the optical power at “off state” the longer it takes for the laser to turn on. One method to reduce laser turn-on time is by not turning the laser off completely when no RF signal is present. The downside of this method is injection of unmodulated light into the fiber cable between RF bursts. Accumulation of this un-modulate light generated by connected optical nodes will eventually leads to saturation of the upstream optical receiver and hence limits the practical maximum number of optical nodes in a RFoG system.
The RFoG technique further has the disadvantage of lower upstream link performance. RFoG optical nodes rely on Amplitude Modulated analog laser for the upstream path. The performance of AM modulated optical transmission systems varies with temperature changes, link distances and laser diode analog performance. The net result of these dependencies is lower overall performance and reduced throughput of the upstream channel.
The RFoG technique further has the disadvantage of optical beating interference. Optical Beating Interference (OBI) is a phenomenon that can occur if multiple optical nodes burst at the same time and their wavelengths are close to each other. The likelihood of OBI increases as the number of optical nodes is increased and whenever upstream traffic generated by CPE units is increased. Existing techniques have attempted to mitigate OBI by deploying two methods. The first method relies on lasers with adjustable wavelength while the second method relies on replacing passive optical splitter with an active splitter. However, with the first method, adjusting the wavelength of each laser in a system limits the maximum number of optical nodes to available wavelength slots that are far apart enough so as not to cause OBI, and thereby essentially limiting the practical number of optical nodes in a system. With the second method, an active splitter entails the use of dedicated optical-to-electrical (OE) conversion hardware for each optical node upstream path. However, every analog OE converter also generates thermal noise that it is added to the converted signal. Accumulation of thermal noise contributed by each OE converter stage essentially limits the maximum number of R-ONU units. FIG. 9 illustrates a RFoG system where the passive 1×N optical splitter 19 has been replaced with 1×N active splitter 23, avoiding OBI by terminating every optical node with an active port and combining the resulting upstream RF signals with analog RF combiners.
The RFoG technique further has the disadvantage of a lack of a management facility. RFoG systems treat the optical nodes as “dumb” devices and therefore do not provide any facilities to manage, monitor or control these optical nodes. This leads to lack of visibility into major parts of the cable TV systems where optical nodes account for most of the active devices in a RFoG network. Furthermore, lack of management facilities leads to lack of means to program the optical nodes and therefore all RF and optical parameters in the optical nodes are hard coded and inflexible. Anytime a different burst detection profile is desired, all optical nodes must be replaced with units that have the new parameters hard coded in their circuitry.